1. Field of the Invention
This invention relates to an acceleration responsive switch mounted on a gas flowmeter having an automatic shutoff valve and an integrated microcomputer and employed in city gas equipment and commercial propane gas equipment or mounted on control devices of oil space heaters, gas burning appliance and electrical equipment, for detecting oscillation such as an earthquake to thereby supply a detection signal to the automatic shutoff valve or the control devices and further relates to a method of making such an acceleration responsive switch.
2. Description of the Prior Art
A seismosensitive element embodying an acceleration responsive switch of the above-described type is disclosed by U.S. patent application Ser. No. 08/121,522 filed on Sep. 16, 1993 and pending. The disclosed seismosensitive element comprises a metal housing, an electrode member fixed in the housing in an insulated relation thereto, and an electrically conductive inertia ball enclosed in the housing so as to roll when subjected to oscillation, so that the inertia ball comes into contact to the electrode member to thereby conduct between the housing and the electrode member, whereby the element generates a detection signal.
The seismosensitive element as described above has recently been mounted on gas flowmeters having an automatic shutoff valve, an integrated microcomputer and an integrated battery in household city gas equipment and commercial propane gas equipment, so that the gas flowmeters are provided with a function of preventing occurrence of secondary disaster such as fire due to an earthquake or a function of early detection of gas leakage as well as a function of storing data of gas flowrate. Such a gas flowmeter detects oscillation and overturning of equipment due to an earthquake, leakage of gas in large quantities and long-term leakage of gas in small quantities, thereby closing the shutoff valve or activating a warning device for prevention of secondary accidents or disasters.
In detection of an earthquake, an oscillation due to the earthquake needs to be distinguished from an oscillation caused by collision of a flying object with the gas flowmeter or an artificial noise caused by driving of a dump truck or the like or under construction. For this purpose, the seismosensitive element is required to have predetermined signal output characteristics in a frequency band of the oscillation due to the earthquake and different signal output characteristics in the other frequency band.
The quake of an earthquake includes a multiple of oscillations with different frequencies. In most cases, the quake accompanies mainly an oscillation whose frequency is 10 Hz or below and more particularly, an oscillation whose frequency is 5 Hz or below. Accordingly, a sinusoidal oscillation is employed as an alternative characteristic for the earthquake for inspection of a seismosensitive device comprising the above-described seismosensitive element. For example, in the above-described acceleration responsive device having contacts closed and opened by the rolling of the inertia ball, each of "on" and "off" periods of a signal is set at 40 milliseconds or above. In this case, when such a signal as described above is generated three times or more within a predetermined period such as three seconds, the microcomputer is arranged to determine the occurrence of an earthquake, thereby delivering a detection signal. The quake of the earthquake is thus distinguished from the disturbing oscillations.
For the purpose of distinguishing the earthquake from the other disturbing oscillations, the seismosensitive element is required to have different response characteristics between a frequency band of the earthquake oscillation and the other frequency band. For example, an oscillational acceleration reaches 120 gal when the sinusoidal wave whose frequency is 5 Hz or below is applied to the seismosensitive element. The acceleration of 120 gal corresponds to the Seismic intensity 5. When such a sinusoidal wave corresponding to the acceleration of 120 gal is applied to the seismosensitive element, the microcomputer is desirable to deliver the detection signal indicative of occurrence of earthquake to thereby operate a safety device such as the gas shutoff valve. Furthermore, the microcomputer is desirable not to make an erroneous response when the acceleration responsive device is subjected even to the acceleration of 300 gal at the frequency of 6 Hz or above.
The conventional control devices such as the gas flowmeter are usually equipped outdoors for the purpose of inspection thereof. For example, the control device is mounted on an outer wall of a building with piping. Accordingly, the control device faces a passage or a playing yard for children. The body of a passenger, his or her luggage, a bicycle or a ball with which children is playing catch sometimes collides against the control device. In such a case, an impulse wave whose acceleration ranges between from 1,000 to 3,000 gal is produced although the acceleration takes different values to some extent depending upon a space between support positions of a metal fixture for the gas piping. Subsequently to the above-described production of the impulse wave, the acceleration starts to be damped from about 1,000 gal. The damped oscillation contains a wave which is approximate to a sinusoidal wave and whose frequency is about 10 Hz. The experiments carried out by the inventors show that such oscillation acceleration as described above is applied to the gas flowmeter.
The signal delivered from the seismosensitive element theoretically has a period in accordance with a period of oscillation applied to the element. Accordingly, when the oscillation frequency is about 10 Hz as in the above-described case, the period of either "on" or "off" signal does not reach 40 milliseconds, so that the microcomputer does not take the oscillation for an earthquake.
However, the inertia ball is enclosed in a circular cylindrical or hemispheric housing as in the above-described seismosensitive element. The inertia ball sometimes rotates along the inner wall of the housing or the electrode when the impulse applied to the element is large. In this case, because of the difference between the frequency of the applied oscillation and a resonance frequency depending upon the elasticity of the electrode, the configuration of the housing bottom and the mass of the inertia ball, the motion of the inertia ball is shifted to an incomplete circular motion resulting in an elliptic or polygonal trajectory in a stage that the oscillation damps. Consequently, the period of the output signal of the seismosensitive element does not always depend upon the frequency of the applied oscillation, or rather, the time length of the "on" or "off" signal corresponds to the condition for determination of an earthquake in the stage that the oscillation damps, which makes it difficult to distinguish the earthquake from the disturbing oscillation.